Single Span HF Test Oscillator

Posted on Feb 5, 2014

This compact RF oscillator covers the entire 0. 4 - 30 MHz range in a single sweep of the dial and has a terminated 50 ohm output of more than 300mV across the entire HF band. Most signal generators use a series of ranges to cover the HF band. This oscillator is a little different. It tunes the entire HF band from 400 kHz to above 30 MHz in a singl

Single Span HF Test Oscillator
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e range. It was designed to test receiver front end designs and HF filters and be compact enough to sit around my workshop bench. The oscillator output level also allows it to be used as a temporary oscillator for testing diode mixers, and it`s sine wave output minimizes harmonics. It`s not possible to directly cover the entire HF band in one range with a traditional LC oscillator. However, mixing an oscillator operating on a higher frequency band with a lower frequency fixed oscillator, it is possible to achieve the required range. This is shown in the design`s schematic in Figure 1. A voltage controlled oscillator (VCO) operates from 48 MHz to 85 MHz. The VCO output (100-150mVpp into 50 ohms) is mixed with the output of a 48 MHz crystal oscillator in a diode mixer to give the required output. The varicap diode is the key to successful wide range VCOs, and the device I used was one of four recovered from an old video recorder tuner. Other wide range varicaps such as Motorola`s MV104 or a Philips BB911 will also work well. The 48 MHz crystal oscillator is typical of those found in equipment such as printers, video cards and the like. These provide a 5V square-wave TTL- compatible output level. I found two plastic encapsulated 48 MHz oscillators in an old Epson printer. The output of the crystal oscillator I used was unable to drive the diode mixer directly, but the series combination of C5 and R3, a 1000pF ceramic capacitor...

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